In an in-line type electron gun is constituted such that three electron beams advance along paths in substantially the same plane. The electrons are emitted from three cathodes and are focused by means of a plurality of electrodes, each having three beam passing holes, ultimately focusing the beams on a screen. A dynamic deflecting magnetic field is established between the electron gun and the screen for determining the landing positions of the beams. The magnetic field is formed by a deflecting yoke to which horizontal and vertical deflecting signals are supplied and which sequentially deflects the beams onto the whole surface of the screen.
When electron beams advance toward the screen after having departed from the electron gun, and are deflected by the deflecting magnetic fields, the electron beams are distorted by non-uniform vertical and horizontal magnetic fields. The distortions are particularly significant when the beams scan the peripheral portions of the screen. Due to such distortions of the electron beams, laterally elongated beam spots may be formed on the screen.
The deflection aberrations are improved by the electron gun which is disclosed in U.S. Pat. No. 4,558,253, and which, as illustrated in FIG. 1, includes a cathode K, a control grid G1, a screen grid G2, and electrodes G3,G4,G5,G6. Particularly, the beam passing planes of the electrodes G5r, G6, which constitute a major lens, are respectively provided with recesses 5R,6R at the centers thereof and three beam passing holes 5H,6H have projecting peripheral rims 5B,6B.
Accordingly, a complex electric field is formed between the electrodes G5r, and G6, and consequently, three major lenses having large diameters and reduced spherical aberrations are formed. The peripheral major lenses are formed in an asymmetrical manner, thereby reducing the deflection aberrations of the peripheral electron beams.
However, in the major lens formed by the electrodes G5r, G6, a focusing difference occurs between the vertical and horizontal directions with the result that the focus quality characteristics are reduced increasing the likelihood of spherical aberrations.
Further, the electrodes G5r, G6 are complicated in their structures, and, therefore, the manufacturing process is difficult. Particularly, the portions requiring a high processing precision, i.e., the beam passing holes and the rims around them undergo plastic deformations during forming in a press with the result that high stresses are concentrated on the portions where the deformations have occurred, thereby reducing precision. Further, depending on the circumstances, partial disruptions can occur, thereby bringing a failure to the manufacturing of the electrodes.